Shadow mastiffs are big canine creatures native to the Plane+of+Shadow, with smooth black coats and mouths full of sharp teeth. On their home Plane, they wander nearly all the time, returning to their lairs only after feeding. On the Material+Plane they seek shelter during the day.
In any condition of illumination other than full daylight, a shadow mastiff can disappear into the shadows. Bright light is fatal to their whelps.
Shadow mastiffs are ruled by the most powerful member of the pack, but there is little loyalty for these neutral evil creatures. They do not speak, but they understand Common.
One theory of their origin is that they're the spirits of animals killed by undead+shadows.